A power split-parallel hybrid electric vehicle (HEV) system is a technique capable of realizing an EV mode, a power split mode, and an over drive (parallel) mode.
Meanwhile, a one-way clutch (OWC) mounted in a vehicle to prevent reverse revolutions of an engine is a device that enables revolutions in a forward direction predetermined in design without limit but restricts revolutions in a reverse direction. If the engine is to be reversely revolved with a strong force, the OWC may be damaged.
A conventional HEV performs control of suddenly reducing the revolution speed of an engine by using an electric motor when the engine is stopped, which is referred to as kill torque control.
This is because, if the engine is stopped due to its inertia, vibration is generated at a specific rpm, thereby providing a driver with an unpleasant feeling. Hence, the kill torque control should be improved.
As shown in FIG. 1, in the graph in which kill torque (TQ) control is performed by using HSG when an engine of an HEV is stopped, the HSG determines TQ to be output according to a revolution speed of the engine, and the TQ to be output is configured as map data through tests in the development of the vehicle.
That is, in engine stop control of the HEV, after the kill TQ control is performed up to a specific engine rpm by using the HSG, kill torque is then removed at the specific engine rpm, and the engine is stopped due to its inertia at the specific engine rpm or less.
Here, a motor that is a power source in the HEV is defined as MG1, and a generator in the HEV is defined as MG2.
However, mode conversion performed by an OWC is impossible in order to reduce the revolution speed down to a low rpm with kill TQ of the MG1, and therefore, the inertia of the engine cannot be offset at the time when the engine is stopped. For this reason, it is difficult to perform the kill TQ control down to the low rpm.
The kill TQ is removed at a high rpm by the OWC of the engine, but reverse revolutions of the engine cannot be thoroughly prevented through the control. Therefore, in a case where the engine has inertia by which the engine can be reversely revolved in a power split-parallel system, the OWC may be damaged due to a strong impact.
In addition, as impact occurs due to the damage of the OWC, the impact of the OWC, if not the impact caused by the inertia of the engine, is transmitted to a driver, which decreases the value of the vehicle.